1. Field of the Invention
The invention relates to an apparatus for accurately measuring the water content of a moist gas containing less than 50 ppm or even less than 10 ppb of water.
2. The Prior Art
Apparatuses for accurately measuring the water content of gases are well known and are frequently referred to as moisture analyzers. One important class of moisture analyzers are those which rely on moisture-induced changes in the electrical resistance of phosphoric acid (P.sub.2 O.sub.5) which is contained within a cell of the analyzer. Such moisture analyzers are described for example in the following U.S.A.
______________________________________ U.S. Pat. No. Hereinafter occasionally referred to as: ______________________________________ 2,830,945 "Keidel '945" 2,900,317 "Keidel '317" 2,993,853 "Berry" 3,146,181 "Bell" 3,147,202 "Johnson" 3,244,602 "Glass" 3,630,875 "Kuffer" 3,799,846 "Capuano" 3,926,745 "Czuha" 4,280,885 "Savery" 4,800,000 "Zatko" ______________________________________
There is no agreement, in the prior art, as to the operating temperature of the cell. Johnson discloses the use of a cooling coil and states that the ". . . temperature . . . preferably is in the range from about 0.degree. to about 50.degree. F. (-18.degree. to 10.degree. C.)". Czuha disagrees and claims a method wherein his "phosphoric acid solution is maintained at a temperature of at least about 50.degree. C.".
In order to maintain the cell, according to Johnson, below room temperature, let us say 20.degree. C., it is necessary to remove heat from the cell. Most of the readily available cooling means however suffer from a lot of drawbacks; in particular the use of tap water requires a source and excludes the use of the analyzer in the field. The use of phase changing refrigerants, like the CFC, suffer from a multitude of drawbacks. All the phase changing products are requiring a compressor in order to change their phase back to liquid for continued use, and the chloro-fluorinated hydrocarbons (CFC) damage the ozone layer of the atmosphere if they escape. Many jurisdictions are considering passing laws restricting their use for cooling purposes.
As is well known in the art the phosphoric acid must be completely dry when first contacted with the moist gas to be measured. It is therefore common practice to cycle the cell by periodically increasing the current through the cell. Examples of this cycling are shown in Bell (FIG. 2), Glass (FIG. 4), Kuffer (FIGS. 1 and 2) and Capuano (FIG. 3). However what the prior art in general and these references in particular have not identified are the problems which occur when a large amount of moist gas is in the system upstream of the cell. These problems include slow cell response times, possible inaccuracies and an inability to measure small moisture concentrations equal to or lower than one part per million.
Accordingly it is an object of the present invention to provide an apparatus for accurately measuring the water content of a moist gas, which apparatus is substantially free from one or more of the problems of the prior apparatuses.
Another object is to provide an improved apparatus which does not employ tap water, nor a phase changing refrigerant, nor a chlorinated hydrocarbon, nor a fluorinated hydrocarbon.
Yet another object is to provide an improved analyzer which has a fast cell response time.
Still another object is to provide an improved analyzer which can measure the moisture content of a gas even down to less than one part per million.